Magnetic sensing devices are an inextri- cable part of the global marketplace for electronic products. Nearly 6 billion
units are shipped each year, and that number
is rapidly growing along with electronics in
general. Magnetic sensors have thousands of
uses, and product designers can choose from
three main types—reed, Hall-effect and magnetoresistive—to provide low-power, high-pre-cision position sensing capability. Each sensor
type has a list of pros and cons, but Honeywell
International, Morristown, N.J., believes its
2014 introduction of a new line of magnetoresistive (MR) sensors, called Nanopower, could
shake the established order.

In the past, MR sensors have faced strong
competition from reed-type and Hall-effect devices, which are often chosen over MR sensors
for reasons of sensitivity and power usage. The
new Nanopower Anisotropic Magnetoresistive
Sensor ICs have an edge, providing sensitivity
that comfortably exceeds Hall-effect devices and
power that finally competes with reed switches.

A new playing field

The Hall-effect switch is the most common
proximity sensor and operates by measuring
the voltage difference, or Hall voltage, across
a conductor. High-gain integrated circuit (IC)
amplifiers packaged with the sensor makes
modern Hall-effect devices relatively compact
and useful in most environments. Drawbacks
of Hall-effect sensors include the need for an
amplifier and susceptibility to magnetic flux.

Reed switches are contained in hermetically sealed glass enclosures. Metal reeds close
contacts in the presence of a magnetic field or
coil. Today, they are often selected for applications where a current leakage-prone solid-state
device, such as a Hall-effect sensors, can’t be
used. Reed switches offer much lower resistance than Hall-effect sensors; and because they
are sealed, they can operate in any environment.

Like Hall-effect devices, MR sensors aresolid-state switches and have no mechanicalparts. Omnipolar, they actuate when the mag-netic field of either the north or south pole ofa magnet is in the sensing range. The sensingdirection for a MR device is in the parallelplane of the IC, which differs from Hall-effectsensors that sense a magnetic field perpendic-ular to the IC. The switch turns off when themagnetic field is removed from the sensor. MRsensors are much more sensitive to magneticfields than Hall-effect, which can increaseflexibility and permit the use of lower-costmagnets. These sensors are also highly reliableand operate in a wide variety of environmentalconditions.Microamp supply current has been thenorm for MR sensor designs until the intro-duction of Honeywell’s Nanopower Series,which includes two sensors, the SM351LT andSM353LT. They draw only 360 nA and 310 nA,respectively, at 1.8 VDC, just one-sixteenth thepower of the best Hall-effect devices. To meetstringent power demands, many applicationstoday utilize a design which power cycles themagnetic sensor by utilizing an external clockand circuitry to turn the system on and off inorder to bring average current through the sys-tem down to less than 500 nA. The new Nan-opower instead uses a clock to internally cyclethe power, simplifying application design.

Magnetic considerations

The discrete IC components needed in Hall-effect transistors to filter electrical noise, such
as that produced by chopper circuits, aren’t
required, allowing a MR sensor to occupy a
small footprint. For magnetic fields between

7 and 14 Gauss, the new sensors can sense
the same magnet target at up to two times the
distance or more when compared to Hall-effect
sensors on the market today. This pays dividends for product design, allowing larger air
gaps between the magnetic target and sensor,
and allows the sensor to be further than a few
millimeters from the target. This sensitivity
can also directly translate to system cost by
allowing designers to use smaller magnets, or
even magnets made of different materials in
their system.

“Due to the significant price increases forrare earth magnets, design engineers usingHall-effect sensors have been looking for waysto decrease the total cost of design by usingless magnetic material, or moving to a morecommon magnet in their applications,” saysJosh Edberg, senior product marketing manag-er for Honeywell Sensing and Control. “Designengineers are also looking for an alternative toreed switches to reduce size and increase qual-ity and durability, while maximizing batterylife.”One other major advantage of the MR sensoris omnipolarity. Unipole Hall-effect sensorsmust actuate with a specific pole of the mag-net, meaning steps to determine the proper

Honeywell’s Nanopower Anisotropic Magnetoresistive

Sensor ICs draw far less current than prior magnetoresistive sensor designs, opening a wider array of applications.
Nanopower is particularly suited to use in portable devices,
and are available in a SOT- 23 package. Images: Honeywell
International